1. Field of the Invention
The present invention concerns an aqueous magnetic fluid composition comprising fine ferromagnetic particles stably dispersed by means of a dispersant in water as a dispersion medium and a process for producing the same. More in particular, the present invention provides an aqueous magnetic fluid composition of high stability in which a dispersant is chemically bonded to the surface of fine ferromagnetic particles by utilizing hydrolysis, as well as an efficient production process therefor.
2. Description of the Relevant Art
Magnetic fluids using water as a dispersion medium in the prior art are described in, for example, Japanese Patent Publication Sho 54-40069 which corresponds to U.S. Pat. No. 4,094,804. In this publication, an aqueous magnetic fluid is produced by first forming on the surface of fine ferromagnetic oxide powder obtained by a wet process, a mono-molecular adsorption layer of a surface active agent comprising an unsaturated fatty acid or salt thereof (a first surface active agent) in an aqueous phase and, subsequently, adsorbing under orientation an anionic or nonionic type surface active agent (a second surface active agent) as the second layer thereby stably dispersing the fine ferromagnetic particles in water.
However, conventional magnetic fluids using water as the dispersing medium involve various problems as described below:
(1) Adsorption of the second surface active agent is an extremely weak physical adsorption (due to van der Waals forces caused by interaction between oleophilic groups of the first and second surface active agents). This phenomenon tends to cause easy desorption making it difficult to maintain stable dispersion of the fine ferromagnetic particles over a long period of time;
(2) The solubility of the second surface active agent is water varies depending on the temperature. Equilibrium between an excess portion present in the bulk and a portion adsorbed to the second layer of the second surface active agent greatly depends on the temperature. That is, the second surface active agent may possibly be desorbed depending on temperature changes to worsen the dispersion stability of the fine ferromagnetic particles;
(3) Since the magnetic fluid itself bubbles readily due to the excess second surface active agent present in water, handling is difficult;
(4) If a small amount of oil or polar solvent is mixed into the magnetic fluid, the surface active agent adsorbed on the second layer or the surface active agent adsorbed on the first layer may cause desorption. As a result, there is a possibility that the dispersed fine ferromagnetic particles will be precipitated or the magnetic fluid per se be emulsified.